The present invention relates to the purification of maleic anhydride recovered from a gaseous reaction mixture produced by the catalyzed partial oxidation of a hydrocarbon. More particularly, the present invention is directed to processes and apparatus for improving the purity and color stability of recovered maleic anhydride.
Maleic anhydride is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It is also a versatile intermediate for chemical synthesis.
Maleic anhydride is prepared commercially by contacting a feed gas comprising molecular oxygen and a suitable hydrocarbon (e.g., n-butane or butene) with a vanadium-phosphorus-oxygen fixed-bed catalyst. The hydrocarbon is partially oxidized as it passes through a tubular reactor containing the fixed bed catalyst. The reaction product gas that is produced contains maleic anhydride together with oxidation by-products such as carbon monoxide, carbon dioxide, water vapor, acrylic and acetic acids and other by-products, along with inert gases present in air when air is used as the source of molecular oxygen.
The prior art discloses a number of methods for isolating and recovering maleic anhydride from the reaction product gas. The preferred method of recovery involves selectively absorbing maleic anhydride in a suitable solvent and then stripping the maleic anhydride from the resulting absorption liquor to obtain a crude maleic anhydride product as described in copending U.S. patent application Ser. No. 08/406,674 (HTS 7798). In the process, the reaction product gas exits the reactor and is cooled to a temperature above the dew point of the water in the reaction product gas in a heat exchanger. The cooled product gas exiting the heat exchanger is introduced into an absorber column in which the gas is contacted with a lean maleic anhydride-absorbing solvent. The gas and solvent come into contact in an absorption zone containing either packing material or trays for promoting gas/liquid contact. Maleic anhydride is absorbed in the solvent along with a portion of the oxidation by-products (e.g., carbon monoxide, carbon dioxide, water, acrylic acid, acetic acid and polymeric tars). The remaining oxidation by-products and inert gases in the product gas pass from the system as an exhaust gas substantially free of maleic anhydride. The absorption liquor exits the absorber column and flows to a rich solvent surge tank.
The absorption liquor from the surge tank is continuously stripped of maleic anhydride in a stripping column to recover crude maleic anhydride and regenerated absorbing solvent containing the absorbing solvent and contaminants. In the stripper column, maleic anhydride and lower boiling materials of the absorption liquor (mostly water and acrylic acid) immediately vaporize. The vapor rises through the column, exits the top of the column and enters a stripper condenser in which the vapor is cooled to produce a condensate. All condensate formed in the stripper condenser is returned to the stripper column as reflux. A liquid side draw of crude maleic anhydride is continuously removed from the stripper column and forwarded to a maleic anhydride purification stage.
Crude maleic anhydride recovered in a process as described above is generally treated by introducing the crude maleic anhydride into a batch distillation column. The batch distillation process consists of a forecut step forecut step followed by a heartcut step. In the forecut step, low boilers are removed at the top of the column and partially condensed. A portion of the condensate is returned to the column as reflux, and the remainder of the condensate is removed as a forecut. After removal of the forecut quantity, the heartcut step is conducted. In the heartcut step, vapor removed from the top of the column is condensed to form a condensate. A portion of the condensate is recycled back to the top of the column as reflux. The remainder is withdrawn as purified maleic anhydride product. The color stability of the maleic anhydride product is improved by increasing the volume of forecut removed from the column. However, removal of a greater volume of forecut reduces the throughput and yield of maleic anhydride product per batch.
U.S. Pat. No. 3,865,849 describes a two stage purification method in which maleic acid in crude maleic anhydride is thermally decomposed into maleic anhydride and water in the first stage, a forerun composed predominantly of water is separated in the first stage, and maleic anhydride is continuously distilled in the second stage. The first stage is carried out in a cascade with at least two reactors in series and a common distillation column operated at 200 mm Hg. Vapors from the reactors are directed to the base of the distillation column. Vapors containing water and low boiling impurities are removed from the top of the column and discharged after condensation as a forerun. After removal of the forerun, maleic anhydride vapors are removed from the top of the column, condensed and returned to the column. Maleic anhydride exiting the last reactor is continuously pumped to a distillation column operated at 60 mm Hg. Vapors withdrawn from the top of the column are condensed. A portion of the condensate is refluxed to the top of the column, and the remainder is withdrawn as purified maleic anhydride. The bottoms residue contains 20 wt. % fumaric acid and high boiling resinous components and 80 wt. % maleic acid. The maleic acid is recovered by distillation and recycled to the first reactor.
U.S. Pat. No. 3,939,183 describes distillation of crude maleic anhydride in a twenty tray distillation column operated at a pressure of about 50 torr and an overhead temperature of about 238.degree. F., a forecut takeoff of 2.5-6% of charge, a heartcut takeoff of 75-85% of charge, and a 9:1 reflux ratio during heartcut takeoff. Improved color stability is obtained by contacting the crude maleic anhydride with a metal halide and phosphorus pentoxide or a perborate before distillation.
U.S. Pat. No. 3,965,123 describes a batch or continuous process for recovering maleic anhydride of good color stability. Crude maleic anhydride from a receiving vessel is introduced into the top of a lights stripper. Vapor taken overhead is recycled to the receiving vessel. Maleic anhydride condenses in the receiving vessel and is returned to the lights stripper. Vapor in the receiving vessel is partially condensed and light boiling materials, such as acetic acid and acrylic acid, are removed. The stripped maleic anhydride exits from the bottom of the stripper and is introduced into a twenty tray batch distillation column operating at 50 mm Hg and an overhead temperature of 238.degree. F. The initial 90 wt % of the material distilled overhead is taken as a product cut. Alternatively, the first 5 wt. % is taken as a forecut and the next 85 wt. % of the overhead stream is taken as a product cut. In another embodiment, the crude maleic anhydride from the receiving vessel is fed directly to the distillation column, a 5 wt. % forecut is taken, and the next 85 wt. % of the overhead stream is taken as product. When manganous chloride is then added to the product and the product is heated at 284.degree. F. for 24 hours, the maleic anhydride has a color of 10-50 Hazen. When manganous chloride is not added to the product, the maleic anhydride has the same or worse Hazen color than typical commercial maleic anhydride.
U.S. Pat. No. 4,260,546 describes a twenty tray batch distillation column whose trays have a separation efficiency of 50% at the conditions of 100 mm Hg absolute pressure, a 1% forecut takeoff and a 1:1 reflux ratio during heartcut takeoff.
U.S. Pat. No. 4,921,977 describes a process for improving the color stability of maleic anhydride by treating crude maleic anhydride with a gas containing oxygen before or during continuous or batch distillation. Crude maleic anhydride is introduced to the tenth plate of a 50 plate fractionating column having an 80 mm diameter and operating at a head pressure of 100 mbar. Vapor removed from the head is condensed and 99% is recycled to the column as reflux. The remaining 1% is removed as product containing 99.98% maleic anhydride. Liquid maleic anhydride taken from the fortieth plate initially has a platinum-cobalt color of 5-10 which increased to 40-60 after two days in storage. When the process is repeated with air being introduced into the crude maleic anhydride fed to the column, the platinum-cobalt color remains at 5-10 after storage.
U.S. Pat. No. 4,961,827 describes a process for preparing purified maleic anhydride having good color stability. Crude maleic anhydride is fed to the eighteenth bubble cap tray of an 80 mm diameter column containing sixty bubble cap trays operated under a head pressure of 150 mbar. Vapor containing low boilers removed from the top of the column is condensed and 99.5% of the condensate is recycled to the column as reflux. The remaining 0.5% of the condensate is product containing 99.97 wt. % maleic anhydride. Vapor taken off from the fiftieth tray is composed of 99.7 wt. % maleic anhydride and 0.3 wt. % citraconic anhydride. The vapor is passed to a condenser in which 2-10% of the gas is condensed as a precondensate. The remaining vapor is completely condensed downstream. The maleic anhydride has a color of less than 40 Hazen when heated at 140.degree. C. for two hours. The precondensate has a color greater than 200 Hazen.
U.S. Pat. No. 5,319,106 describes a maleic anhydride purification process in which crude maleic anhydride is fed from a stripper to a light ends distillation column. A polymerization inhibitor is fed to the top of the column to inhibit polymerization of acrylic acid. Acrylic and acetic acids are distilled overhead and vented. The bottoms from the stripper are introduced into a product distillation column wherein a vapor is distilled off overhead and condensed to form a condensate. The noncondensables are vented, and the condensate is the maleic anhydride product. The bottoms from the column, which contain heavy ends, the polymerization inhibitor and organic solvent, are recycled to the stripper.
British Patent. No. 1,291,354 describes purification of crude maleic anhydride by feeding the crude to the fifteenth tray of a distillation column having twenty trays operating at a pressure of 70 mm Hg. Purified maleic anhydride is removed as a vapor above the fifteenth tray. A forecut of about 0.5% of the crude is withdrawn at the top of the column.
European Patent Application No. 612,714 describes a continuous maleic anhydride purification process in which crude maleic anhydride is introduced into a first fractionating column wherein light boiling impurities are removed as an overhead vapor and liquid maleic anhydride is removed from the bottom of the column and directed to the center stage of a second fractionating column. The overhead vapor is partially condensed and the maleic anhydride condensate is totally recycled to the column. The uncondensed vapor containing maleic anhydride, water and coloring substances is partially condensed in a second condenser and the condensate is recycled upstream to the absorption tower or the first fractionating column. Purified maleic anhydride containing less than 0.01% acrylic acid and less than 0.05% maleic acid is recovered from a stage above the center stage of the column.